Feline hippocampal pyramidal cells were studied with compound multibarrel micropipettes using the technique of combined penicillin 8PCN) and amino acid iontophoresis. PCN iontoporesis increased the unit firing rate in most cases, and decreased the duration of electrically induced post-discharge inhibition. Conversely, following topical PCN-induced interictal surface discharges, unit firing was inhibited for a longer duration (2-10sec) than following electrically induced post-discharge inhibition (50-370msec). GABA suppressed D-1-homocysteic acid induced and spontaneous unit firing, but failed to completely suppress topical PCN-induced unit firing occurring between surface interictal discharges.